Instrumentation sensors that operate on a principle of vibration of constrained actuator masses are known in the art. Angular rate gyroscopes make use of the principle of inertia to measure the rate of rotation through an angle with respect to a sensing axis. One type of angular rate gyroscope is the solid-state gyroscope. Vibrating rate gyroscopes utilize standing waves that are excited in a resonating element to produce a desired mode of oscillation having a predetermined number of nodes at predetermined node spaces. The oscillations have an inherent oscillatory inertia that is insensitive to the translation motion of the gyroscope itself, as well as to rotational movement that is orthogonal to a sensing axis. To the extent the resonating element is rotated about the sensing axis, the oscillations will essentially maintain their inertial orientation (i.e. the predetermined number of nodes and node spacing). The rotation of the nodes that define the desired mode of oscillation may lag the actual rotation of the physical structure of the resonating element about the sensing axis. The lag is characterized by a “precession constant” that is the ratio of the rotation of the oscillation pattern to the rotation of the resonating element. Accordingly, it is possible to determine the rate of rotation of the resonating element, in addition to the magnitude and direction of rotation by measuring the rotational displacement of the nodes on the resonating element.
Solid-state gyroscopes based on the principle described above are capable of sensing only rotation and then usually only about a single axis. To obtain information sufficient to determine the relative attitude of a body, it is necessary to group three such gyroscopes in an orthogonal relationship covering the x, y, and z Cartesian axes. The inherent challenge in using vibrating rate sensors, particularly multiple vibrating rate sensors, is trying to reduce or cancel out any differences or noise at the nodes so that the accuracy and reliability of the solid-state gyroscope can be optimized.
The conventional wisdom is that to minimize the inherent challenges presented by vibrating rate sensors a gyroscope should have maximum geometrical symmetry among the rate sensors, be made of materials having a high mechanical “Q” (defined as ratio of the magnitude of the total energy of a vibrating system to the magnitude of the energy added to the system during each oscillatory cycle), and feature isolation of the drive and rate sensing functions. See, e.g., W. S. Watson, Vibrating Element Angular Rate Sensor For Precision Applications, IEEE Position Location and Navigation Symposium, 1990; D. D. Lynch, Coriolis Vibratory Gyros, Symposium Gyro Technology, Stuttgart Germany, September 1998; and C. Fell, I. Hopkin, K. Townsend, A Second Generation Silicon Ring Gyroscope, Symposium Gyro Technology, Stuttgart Germany, September 1999. Unfortunately, these aspirations must be regularly compromised in the real world in order to reduce cost and complexity of the gyros.
For example, one type of angular rate sensor utilizes a cup or bell shaped sensor that is supported upon a stem and secured to the chassis of the sensor. The surface of the cup comprises drive electrodes and sense electrodes that are alternately oriented symmetrically around the perimeter surface. Exciting the drive electrodes induces a controlled oscillation upon the cup. The sense electrodes produce a signal that is demodulated in control circuitry to determine the angular rate at which the sensor is rotated. A number of techniques are known in the art that attempt to correct for errors in the desired mode of oscillation due to inherent errors and imperfections in the gyroscope assembly.
U.S. Pat. No. 5,471,875 to Sato et al. teaches the use of a first pair and a second pair of radially opposed driving electrodes provided on the outer surface of a resonator located at a pre-determined antinode axes of a cylindrical shaped resonator, and means for generating concurrently a first force and a second force at each of the first driving electrodes and each of the second driving electrodes, respectively, so that the first force reverses its direction along the radius of the resonator at a regular interval, the second force reverses its direction along the radius of the resonator at a regular interval, and the direction of the first force is opposite the direction of the second force. The complimentary action of the first and second forces prevents or limits offset of the nodes thereby restricting the null voltage signal at the sensors, which are located at the oscillation nodes. Accordingly, the drive and the sense electrodes are distributed at equal angular spacing about the centerline of the cylindrical resonator and are coincident with the antinodal and nodal axes, respectively.
U.S. Pat. No. 5,218,867 to Varnham et al. and U.S. Pat. No. 6,805,007 to Fell, et al. disclose what is herein referred to as a “mixed pair” of elements, i.e. drive elements diametrically opposed to sense elements about axisymmetric vibratory elements. At least two such mixed pairs are utilized, with a rotational displacement of 45° therebetween. The 45° displacement coincides with the spacing between adjacent nodal and antinodal axes of the disclosed oscillatory patterns. The angular arrangement about the centerline enables the drive elements to control the oscillation pattern and optimizes sensing of the position of the oscillation pattern in response to a rotational rate.
U.S. Pat. No. 5,445,007 to Varnham et al. discloses a correction technique that entails splitting the connections for one of the drive electrodes and driving the split electrode with a pair of drive voltages that are then typically detected by a corresponding pair of drive sensors for purposes of making adjustments to attempt to correct any errors in the desired mode of oscillation. This technique suffers from a number of shortcomings. In order to maintain symmetry of the oscillation, the mass and size of all of the elements are matched as closely as possible, but the asymmetry introduced by splitting one drive electrode adversely affects the overall uniformity and ability to maintain the desired oscillation mode. Additionally, the split drive electrode requires two conductor connections instead of the single conductor utilized on the other elements. The additional mass of the second connection introduces further asymmetry that is deleterious to resonance performance. In addition, the split drive plate is used only for static alignment, and is not used to augment active torque adjustment.
Compensation techniques exist to counter these shortcomings, such as corrective signals from other sensors such as thermistors, the use of EEPROM correction tables, or restricting use to a reduced temperature range. However, these measures tend to add complexity and cost to the design and require extra testing and adjustment. Even if cost and complexity were not issues, these measures are of limited effectiveness as they attempt to correct the problem after the fact, rather than addressing the underlying challenge.
Another known type of angular rate sensor comprises the use of piezoelectric ceramic bender elements in a paired tuning fork arrangement. In this type of arrangement, a pair of drive elements is energized to induce a controlled vibration within a single plane. The application of rotational forces upon the vibrating elements parallel to the plane of vibration and on the axis of symmetry induces a measurable signal characteristic of the angular relationship between the sensing object and the vibrating elements. Inherent to tuning fork designs are the bending forces that result from the oscillating drive elements. Although some designs attempt to reduce such undesirable forces by isolating the drive and sense elements, there are still errors that lead to reduced signal-to-noise ratios and false indication of rotation.
While numerous vibration based angular rate sense systems exist, none provide a simple and economical design with the ability to both adjust the angular relationship between the drive and sense elements and maintain symmetrical mass structures for maintaining desired oscillatory modes.